diff --git a/js/src/Makefile.in b/js/src/Makefile.in
index d228cc4a4b7..60e378cf3ea 100644
--- a/js/src/Makefile.in
+++ b/js/src/Makefile.in
@@ -83,6 +83,10 @@ CSRCS		= \
 		prmjtime.c \
 		$(NULL)
 
+ifeq ($(MOZ_OS2_TOOLS),VACPP)
+ASFILES		= jslocko.asm
+endif
+
 EXPORTS		= \
 		jsautocfg.h \
 		js.msg \
diff --git a/js/src/jslock.c b/js/src/jslock.c
index 87b7b3ffc49..59ad77fa055 100644
--- a/js/src/jslock.c
+++ b/js/src/jslock.c
@@ -149,6 +149,10 @@ js_CompareAndSwap(jsword *w, jsword ov, jsword nv)
     return !_check_lock((atomic_p)w, ov, nv);
 }
 
+#elif defined(XP_OS2_VACPP)
+
+/* js_CompareAndSwap implemented in jslocko.asm */
+
 #else
 
 #error "Define NSPR_LOCK if your platform lacks a compare-and-swap instruction."
diff --git a/js/src/jslock.h b/js/src/jslock.h
index ef973e00744..1d4fe226747 100644
--- a/js/src/jslock.h
+++ b/js/src/jslock.h
@@ -200,7 +200,7 @@ extern JSBool js_IsScopeLocked(JSScope *scope);
     !( (defined(_WIN32) && defined(_M_IX86)) ||                               \
        (defined(__GNUC__) && defined(__i386__)) ||                            \
        (defined(SOLARIS) && defined(sparc) && defined(ULTRA_SPARC)) ||        \
-       defined(AIX) )
+       defined(AIX) || defined(XP_OS2_VACPP) )
 
 #define NSPR_LOCK 1
 
diff --git a/js/src/jslocko.asm b/js/src/jslocko.asm
new file mode 100644
index 00000000000..463c07dadf5
--- /dev/null
+++ b/js/src/jslocko.asm
@@ -0,0 +1,60 @@
+COMMENT | -*- Mode: asm; tab-width: 8; c-basic-offset: 4 -*-
+***** BEGIN LICENSE BLOCK *****
+Version: MPL 1.1/GPL 2.0/LGPL 2.1
+
+The contents of this file are subject to the Mozilla Public License Version 
+1.1 (the "License"); you may not use this file except in compliance with 
+the License. You may obtain a copy of the License at 
+http://www.mozilla.org/MPL/
+
+Software distributed under the License is distributed on an "AS IS" basis,
+WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License
+for the specific language governing rights and limitations under the
+License.
+
+The Original Code is an OS/2 implementation of js_CompareAndSwap in assembly
+
+The Initial Developer of the Original Code is
+IBM Corporation.
+Portions created by the Initial Developer are Copyright (C) 2001
+the Initial Developer. All Rights Reserved.
+
+Contributor(s):
+
+Alternatively, the contents of this file may be used under the terms of
+either the GNU General Public License Version 2 or later (the "GPL"), or
+the GNU Lesser General Public License Version 2.1 or later (the "LGPL"),
+in which case the provisions of the GPL or the LGPL are applicable instead
+of those above. If you wish to allow use of your version of this file only
+under the terms of either the GPL or the LGPL, and not to allow others to
+use your version of this file under the terms of the MPL, indicate your
+decision by deleting the provisions above and replace them with the notice
+and other provisions required by the GPL or the LGPL. If you do not delete
+the provisions above, a recipient may use your version of this file under
+the terms of any one of the MPL, the GPL or the LGPL.
+
+***** END LICENSE BLOCK *****
+        |
+
+        .486P
+        .MODEL FLAT, OPTLINK
+        .STACK
+
+        .CODE
+
+;;;---------------------------------------------------------------------
+;;; int _Optlink js_CompareAndSwap(jsword *w, jsword ov, jsword nv)
+;;;---------------------------------------------------------------------
+js_CompareAndSwap     PROC OPTLINK EXPORT
+        push ebx
+        mov ebx, eax
+        mov eax, edx
+        mov edx, ebx
+        lock cmpxchg [ebx], ecx
+        sete al
+        and eax, 1h
+        pop ebx
+        ret
+js_CompareAndSwap     endp
+
+        END